The present invention relates generally to the production of x-rays and other energetic electromagnetic radiation (short wavelengths), and more specifically to techniques for interacting relativistic electrons with electromagnetic radiation having relatively long wavelengths to generate electromagnetic short-wavelength radiation.
The unique ability of electron beam-based sources of electromagnetic radiation employing undulators to generate intense, near monochromatic, forward peaked beams of radiation have made undulators critical components of advanced light sources such as second and third-generation synchrotron radiation sources and free-electron lasers. There are therefore many references to undulator technology and the use of undulators in the literature, beginning with Motz' pioneering description of the concept and first demonstration at Stanford (Motz 1951) to the many published descriptions of the concept and its implementation in connection with the development of the free-electron laser (Madey 1971) and the second generation synchrotron radiation sources at Brookhaven National Laboratory (Decker 1996), Lawrence Berkeley Laboratory (Robinson 1991), the Stanford Linear Accelerator Center (Hettel 2002) and Argonne National Laboratory (Galayda 1995).
Almost all such systems constructed to date employ undulators constructed as a linear array of dipole magnets designed to create a static, transverse, spatially periodic magnetic field in which the magnetic component of the Lorentz force ev×B imposes both a periodic transverse acceleration and a periodic transverse velocity on the motion of the electrons moving through the field. Typical magnet periods range from somewhat less than a cm to the order of 10 cm depending on the wavelength of radiation desired and the energy of the electron beams available for use in the system. To maximize the radiated power while limiting emission at the harmonics, these systems are typically operated at normalized vector potentials an of order between 0.1 and 1.0. Typical undulator lengths range from 1 to 10 meters as required to achieve the desired spectral bandwidth. As an example, an undulator operating at with an232 0.2 designed to produce x-rays of 10 angstroms wavelength with a spectral bandwidth of 1% at an electron energy of 3.0 GeV and an electron beam with minimal angular divergence would have a period of 5.7 cm and a length of 3 meters.
The extended length of the undulators used for such systems, together with the size, cost and complexity of the accelerator systems needed to generate the high energy, high power electron beams required for operation, have made such light sources both physically large and expensive. As examples, the X-ray light sources at Brookhaven, Lawrence Berkeley Laboratory, Stanford, and Argonne have, respectively, diameters of 54, 63. 75, and 350 meters with construction costs ranging from $160 million to $500 million.
A related physical phenomenon, inverse Compton scattering, has also been investigated as a means for production of short wavelength electromagnetic radiation in synchrotron radiation sources (Ruth 1998, Ruth 2000, and Harteman 2004) and free-electron lasers (Elias 1979). The inverse-Compton mechanism combines two basic physical effects, Compton scattering in which an incident electromagnetic wave is scattered by a single electron, and the Doppler shift, in which the radiation emitted by moving charges is upshifted in frequency along the direction of motion.
However, the concept of Compton scattering as described in the literature (Heitler 1960) is applicable only when the mechanism can be described as the scattering of single photons, and is no longer valid when the electric and magnetic fields of the incident electromagnetic wave are strong enough to induce transverse velocities approaching the speed of light, e.g., when their normalized vector potential approaches unity. Given this restriction to low field amplitudes and the dependence of the radiated power on the square of the field amplitude, electron beam-based inverse-Compton light sources have simply not proven competitive with undulator-based light sources to date.